1. The present invention relates to polymer filtration systems and, more particularly, to an improved method and apparatus for replacing clogged filter sections.
2. Discussion of the Prior Art
In extruding articles of small cross-section from thermoplastic polymers such as polyethylene, Nylon, polyester, polystyrene, etc., it is necessary to filter foreign material from the molten polymer. Two common examples of such articles are synthetic textile fibers and thin plastic films of the type used in packaging or for tapes (e.g., computer tape, sound recording tape, etc.) In the production of synthetic textile fibers, which may have a final diameter of as little as ten microns, a particle of foreign matter of five or more microns diameter is quite likely to cause breakage of the fiber during manufacturing. It is desirable, therefore, to filter out any foreign material above a certain size.
In the extrusion of pigmented fibers, the pigment particles may agglomerate to a sufficient size to cause fiber breakage. It is also desirable to filter out these excessively large particles, even though they are deliberately added to the polymer and are not, therefore, truly foreign material.
"Screen Changers" having a reciprocating plate or ram with two flat screen filter elements are well known in the prior art. An example of such a device may be found in U.S. Pat. No. 3,007,199 (Curtis). This type of filter is not suitable for fiber or thin film production because air is introduced into the molten polymer stream when a clean screen is shitted into place. The introduced air tends to cause holes in the extruded film, or breaks in extruded fibers, thereby interrupting smooth operation of the manufacturing process. In order to avoid this problem it is necessary to use a so-called "continuous" polymer filter wherein the polymer stream is not interrupted and no air can be introduced when clean filter media are brought into the polymer stream.
There are four general types of prior art continuous polymer filters that have achieved some commercial success, although each type has certain inherent problems and disadvantages. These four types of filters are: (a) the dual cartridge filter; (b) the dual ram filter; (c) the screen belt filter, sometimes called Kalman filter; and (d) the moving breaker plate filter. Each of these filters, as well as their advantages and disadvantages, is described in the following paragraphs.
Dual cartridge filters are disclosed in U.S. Pat. Nos. 3,940,222 (Zink); 3,503,096 (Marianelli); 4,202,659 (Kinoshita); 4,511,472 (Trott); and 3,669,166 (Colin). In addition, a dual cartridge filter is manufactured and sold by the Fluid Dynamics Company of Cedar Knolls, N.J. as the CPF System. Filters of this type generally include two small pressure vessels operating at working pressures of at least 3,000 psig. Each vessel contains one or more re-usable filter elements and some valve mechanism (generally two plug valves) in order to permit diversion of polymer flow from one vessel to the other. A vent, or the like, is provided at the high point of each vessel to bleed out all air when polymer is re-introduced into a vessel after its associated filter element has been cleaned. Dual cartridge filters normally require several minutes to remove a dirty element from the vessel from which polymer flow has been diverted, necessitating that all polymer be directed through the other element while the dirty element is being cleaned. The main advantage of dual cartridge filters resides in the fact that they permit each filter element to incorporate a very large area, thereby permitting very fine filter media to be employed (i.e., five or ten micron screen) without excessive pressure differential being developed across the filter. On the other hand, this type of filter has three main disadvantages, namely: (1) it is expensive to manufacture; (2) the filter elements are too costly to be disposable and therefore must be cleaned at considerable expense; and (3) the pressure differential across the filter rises markedly as one set of filter elements gets dirty, and then drops dramatically as clean elements are brought on stream. The change in pressure differential normally causes the extruder exit pressure to rise and fall, thereby varying the degree of polymer shearing and causing variations in the final product being extruded.
A dual ram filter is disclosed in U.S. Pat. No. 4,167,384 (Shirato et al). In addition, a dual ram filter is manufactured and sold by Maschinenfabrik Joachim Kreyenborg and Co. of West Germany as the K-SWE model continuous operating screen changer. These filters use simple flat pieces of woven screen wire as the filter medium and have two reciprocating rams that are similar to the single ram disclosed in the aforementioned Curtis patent. Each ram has its own screen and breaker plate, and both screens are in flow communication with both the extruder and the filter exit port. In order to change a dirty screen for a clean one, one of the rams is moved so as to bring the breaker plate region of the ram outside the filter body, permitting the machine operator to manually remove the screen, insert a new piece of screen, and then retract the ram into the body. The system includes means for filling the cavity on the upstream side of the screen with polymer, and means for bleeding out all of the air before the ram is moved to the full inward running position. Both rams are positioned within the body except for the short time interval required to change a screen. In this way no polymer solidifies on the breaker plate and it is not necessary to clean (i.e., "burn out") a breaker plate as required with the moving breaker plate type filter described below. The so-called "Kreyenborg ram" is provided with a very close fit within the body, thereby eliminating the need for seals utilized in the system disclosed in the aforementioned Curtis patent. The main advantage of the dual ram filter is its use of inexpensive flat woven screen and the absence of plates to be burned out. Disadvantages include: limited screen area; the likelihood of a ram getting stuck in the body and not moving freely; difficulty in sizing the bleed ports if different polymers of widely different viscosity are to be processed; large variations in pressure differential across the filter as a ram is shifted to change a screen; and high labor cost resulting from the need for an operator to be employed to change the screens which must be changed relatively frequently because of the requirement for a small screen area (relative to the dual cartridge filter).
Screen belt filters, sometimes referred to as "Kalman filters", are disclosed in U.S. Pat. Nos. 3,471,017 (Kalman); 3,645,399 (Kalman); 3,856,674 (Kalman); 3,940,335 (Kalman); and 4,238,877 (Rapp). A woven screen wire belt, one hundred feet or more in length, passes through a steel block normally bolted to the end of an extruder. The belt travels at right angles to the direction of polymer flow and is supported by a stationary breaker plate. Sealing devices surround the belt at the locations where the belt enters and leaves the block. Cooling of the sealing devices may be selectively effected via water or other liquid passing through flow paths defined in each sealing device. Electric heaters are provided to momentarily heat the sealing devices and thereby permit advancement of the belt in the manner described in the Kalman patents. Briefly, the belt exit slot is considerably thicker than the inlet passage, causing the polymer to leak at a more rapid rate from the exit passage when the sealing blocks are heated and thereby causing the screen to move along with the leaking softened polymer. The Kalman patents also disclose mechanical pulling arrangements for moving the screen, instead of relying solely on polymer leakage. It is also suggested in the Kalman patents that movement of the screen obliquely, relative to the polymer flow, can assist in screen movement while increasing the area of screen available for filtration. In operation, the inlet and exit seals are alternately heated and cooled to cause intermittent screen movement. While the Kalman patents indicate that it is possible to control the inlet and outlet seal temperatures to achieve steady, continuous movement of the screen, in practice it is quite difficult to accomplish this at the very slow rates required. Typically, these screen belt filters utilize a timer to turn the heat and cooling water on and off, resulting in small (e.g., perhaps one-tenth of the belt width) intermittent movements. The screen belt filter has many advantages: it is simple and inexpensive to manufacture; it functions totally automatically, requiring very little attention by the machine operator; movement of the screen in small increments results in only minor changes in the pressure differential across the filter; the screen is not too expensive and can be discarded; no "burn out" of breaker plates is required; and it has no moving parts except for the screen belt itself. The main disadvantage of the belt screen filter is its lack of suitability for filtering fine particles; therefore, these filters are generally used with screens having a filter rating of sixty to one hundred fifty microns. If a finer screen is employed, the pressure drop across the screen is so great that friction between the screen and the stationary breaker plate makes it impossible to advance the screen without causing screen failure due to excessive tension. A filter can be made for a screen of any width, but the breaker plate usually cannot be more than approximately 3.0 inches in the direction of screen movement, regardless of the width of the plate. This means that the breaker plate area increases only as the first power of the belt width, whereas the output flow from an extruder varies roughly according to the square of the screw diameter. This fact has limited the success of screen belt filters to smaller extruders, or to coarse filtration on a larger extruder (e.g., in excess of 4.5 inch screw diameter).
It must be stressed that, in spite of the popularity of the Kalman-type filter, there has been no solution in the prior art to the problem concerning the unsuitability of the filter with fine screen material for filtering fine particles. In fact, subsequent to obtaining his basic U.S. Pat. No. 3,471,017, Kalman obtained three additional U.S. Pat. Nos. 3,645,399; 3,856,674; and 3,940,335, all directed toward solving the same problem using the basic filter disclosed in U.S. Pat. No. 3,471,017. Kalman admits in U.S. Pat. No. 3,856,674 that, as the filter becomes progressively clogged with impurities, the force exerted upon the active part of the filter, resulting from the hydrostatic pressure on the substance being filtered, can rise to such an extent that, in cases of heavy contamination, it is extremely difficult to move the filter screen. Actually, the problem is not limited to heavy contamination. If a fine screen (e.g., ten to thirty microns) filter is employed for a high flow rate viscous polymer (e.g., five thousand pounds per hour of polymer at four thousand poise), an enormous Kalman filter assembly would be required, even if the polymer contains a very low level of contamination, because the pressure drop through a perfectly clean screen would be extremely high. This is a direct result of the requirement that the breaker plate area must be narrow in the belt movement direction; if it is not, the differential pressure renders belt movement virtually impossible.
None of the filters disclosed in the three aforementioned improvement patents of Kalman has achieved any commercial success, although the original Kalman filter continues to have wide acceptance on small extruders using screens no finer than sixty microns. It is believed that the absence of commercial success for the improvements results from the fact that all of the improvement devices disclosed by Kalman require complicated mechanical devices.
The other major type of continuous filter is manufactured by the Berlyn Corporation of Worcester, Mass. and sold under the model name Berlyn Continuous Filter Model CF3539. The so-called Berlyn filter functions in a manner somewhat similar to the Kalman screen belt filter in that seals at the inlet and outlet of the filter are kept cool by water, preventing polymer leakage by freezing any polymer tending to escape. With the Berlyn filter, a flat woven filter screen is supported by a heavy breaker plate pushed intermittently through the filter body by a large hydraulic cylinder. The seals are maintained at a steady temperature, and plate movement is in very small (e.g., 0.010 inch) and frequent steps (e.g., on the order of one per minute). Relative to the Kalman filter, the Berlyn filter offers two major advantages. Since the screen and breaker plate move together, there is no tensile force applied to the screen to initiate movement. This permits much finer screens to be employed. Also, there is no restriction against the polymer flow region of the screen having an approximately square configuration, or even a configuration which has a greater dimension in the direction of plate movement than the belt width dimension. Therefore, a moving breaker plate filter having, for example, a screen eight inches wide, is capable of having an opening of eight inches by ten inches or eighty square inches of area. A Kalman-type filter, on the other hand, would be limited to an opening of eight inches by three inches, for a total area of twenty-four square inches. The Berlyn filter is, therefore, more suitable for use on a large extruder or in applications requiring fine filtration. The main disadvantages of the Berlyn-type moving breaker plate filter relative to the Kalman-type filter are much higher manufacturing costs and the requirement to "burn out" and clean a breaker plate that emerges from the exit side of the filter assembly before that breaker plate can be fitted with clean screen material and re-inserted at the inlet end of the assembly.
In addition to the four types of filters described above, there are some backflush-type polymer filters in commercial use. These filters generally utilize a flat woven screen which is not changed but is backflushed periodically with polymer. Backflush filters are not too popular because they waste polymer in flushing, and because the flushing action is not altogether successful in removing very fine particles from very fine screen.